In vehicles, there is a hybrid vehicle in which an engine driven by combustion of fuel is directly connected to an electric motor driven by electric power and having a power generating function. In this hybrid vehicle, torque is output from the engine and/or the electric motor and is transmitted to vehicle drive wheels through a fluid torque converter and a transmission. That is, in the vehicle drive system the motor in connection with a crankshaft of the engine assists the output of the engine, and a controller controls the assist torque in combination with the automatic transmission equipped with the fluid torque converter.
Conventionally, the hybrid vehicle is provided with not only the engine but the electric motor (herein “motor”) to improve fuel efficiency (see JP Application Laid-Open No. H09-84210 and JP Application Laid-Open No. 2000-197209). In this hybrid vehicle, the motor assists the drive torque if the engine load is heavy or the torque ratio of the torque converter is large, whereas the motor does not assist the drive torque if the engine load is light or the torque ratio of the torque converter is small, so that the motor is driven frequently in a range the efficiency of the engine is relatively high.
However, there are some problems if the motor assists the drive torque under a condition where the engine load is heavy or the torque ratio of the torque converter is large.
That is, under a condition where the engine load is heavy, the torque that the automatic transmission receives, i.e., transmission input torque, becomes large. As shown in FIG. 7, if the ratio “e” between input and output speeds of the torque converter (turbine rotational speed/pump rotational speed) is small, the transmission input torque becomes larger due to the fact that the torque is multiplied by the torque converter before the coupling point. In other words, the torque converter has efficiency “n” increased due to movement of a stator, thereby increasing torque ratio “t”. If the electric motor assists the engine under this condition, the transmission input torque may exceed a permissible value or capacity, thereby reducing durability of the automatic transmission.
To obviate this problem, the vehicle is typically provided with an automatic transmission having a large permissible value or capacity for receiving the input torque. However, the larger the permissible capacity for input torque, the heavier and larger the automatic transmission becomes, and the greater the torque lost owing to the friction in the automatic transmission, which is disadvantageous in improving fuel efficiency.